This invention generally relates to radar techniques for determining angular location of a target and, more particularly, to an improvement in the monopulse technique so as to maintain accuracy of the monopulse ratio in the presence of jamming by adaptively and optimally suppressing the jamming before forming the conventional sum .SIGMA. and difference .DELTA. beam output signals for monopulse processing.
The monopulse technique is a radar technique in which the angular location of a target can be determined within fractions of a beamwidth by comparing measurements received from two or more simultaneous beams. This technique for direction of arrival (DOA) estimation of a target is widely employed in modern surveillance and tracking radar. See, for example, D. K. Barton, "Modern Radar Systems Analysis," Artech House (1988), M. Sherman, "Monopulse Principles and Techniques," Artech House (1988), and I. Leanov and K. I. Fomichev, "Monopulse Radar," Artech House (1986). In a typical phased array or digital beamforming (DBF) radar, one beam is formed in transmission and two beams are formed on reception for angle measurement.
The monopulse technique may be implemented for a linear array of N antenna elements which provide respective signals x (0), . . . , x (N-1) to the beamforming network from the elemental receiver. The output signals of the beamforming network are the sum .SIGMA. and difference .DELTA. signals which are processed in a processor to generate an output signal .theta. representing the direction of arrival estimation.
In the beamforming network, each of N input signals is split into two paths, linearly weighted, and then added together. The sum .SIGMA. and difference .DELTA. signals may be expressed in the form EQU .SIGMA.=W.sub..SIGMA..sup.H x (1) EQU .DELTA.=W.sub..DELTA..sup.H x (2)
respectively, where W.sub..SIGMA. is real and even weighting, W.sub..DELTA. is purely imaginary and odd weighting, H indicates complex conjugate transpose and x is the vector of the measurements. When there is no jamming, Taylor and Bayliss weightings are typically used for sum beams and difference beams, respectively, so as to have a narrow mainlobe and low sidelobes. In the presence of jamming, the weights are adapted so as to form nulls responsive to the jammers. The quiescent Taylor and Bayliss weightings are designed for reducing the sidelobes in a practical system. See Y. T. Lo and S. W. Lee, "Antenna Handbook, Theory, Applications, and Design", Van Nostrand Reinhold Company, New York (1988), Chapter 13.
In a typical antenna pattern, the mainlobe of the pattern is a central beam surrounded by minor lobes, commonly referred to as sidelobes. Typically, it is desired to have a narrow mainlobe, high gain and low sidelobes so that the desired target within the mainlobe is enhanced and the response to clutter and jamming outside the mainlobe is attenuated. The sidelobe levels of an antenna pattern can be described in any of several ways. The most common expression is the relative sidelobe level, defined as the peak level of the highest sidelobe relative to the peak level of the main beam. Sidelobe levels can also be quantified in terms of their absolute level relative to isotropic.
The term "monopulse" refers to the fact that the echo from a single transmitted pulse returning from a target is used to measure the angle of the target, and that, typically, one beam (instead of two beams) is formed in transmission, and two beam output signals are formed on reception for angle measurement. The sum beam pattern has a symmetrical amplitude profile with its maximum at the boresight, and the difference beam pattern has an antisymmetrical amplitude profile with zero response at the boresight. The DOA of a target signal can be determined accurately through a look-up table by evaluating the monopulse ratio, i.e., the real part of .DELTA./.SIGMA.. In fact, for a noiseless case and for uniform weighting, the monopulse ratio is exactly given by ##EQU1## where T =sin (.theta.) and .theta. is the desired DOA, d is the array element spacing, N is the number of sensor elements, and .lambda. is the wavelength. This equation enables T and the corresponding .theta. to be determined exactly. In the presence of noise, the development of the DOA maximum likelihood estimator also leads naturally to monopulse processing using sum and difference beams. See R. C. Davis, L. E. Brennan, and I. S. Reed, "Angle Estimation with Adaptive Arrays in External Noise Field," IEEE Trans on Aerospace and Electronic Systems, Vol. AES-12, No. 2, March 1976. For zero-mean noise, the estimator is unbiased with mean square error (MSE) given by ##EQU2## SNR is the signal-to-noise ratio at the elemental level, and g (T) is the two-way sum beam antenna pattern.
Various authors have defined the monopulse sensitivity factor in different ways (see R. R. Kinsey, "Monopulse Difference Slope and Gain Standards," IRE Trans., Vol AP-10, pp. 343-344, May 1962). In this application, the monopulse sensitivity factor is defined as the constant of proportionality required in the denominator of the root-mean-square-error (RMSE) to convert the square root of twice the boresight signal-to-noise ratio in the beam to RMSE. Defined in this manner, the monopulse sensitivity factor has the desirable property of containing all target angle-of-arrival information. "f" is the monopulse function and "f dot" is the derivative of the monopulse function. See D. J. Murrow, "Height Finding and 3D Radar", Chapter 20, Radar Handbook (2nd Edition), McGraw-Hill.
This technique can also be considered for a planar array where the target azimuth and elevation angles are desired. In this setup, a set of sum (.SIGMA..sub.e) and difference (.DELTA..sub.e) beam output signals are formed along the elevation axis with input signals from each column of sensors. The .SIGMA..sub.e beam output signals are then linearly combined in an adder to form the sum (.SIGMA.=.SIGMA..sub.a .SIGMA..sub.e) and difference (.DELTA..sub.A =.DELTA..sub.a .SIGMA..sub.e) beam output signals along the azimuth axis, where .SIGMA..sub.a and .DELTA..sub.a are the effective row sum beam and row difference beam, respectively. Similarly, the .DELTA..sub.e beams are linearly combined in an adder to form the sum (.DELTA..sub.E =.SIGMA..sub.a .DELTA..sub.e) and difference (.DELTA..sub..DELTA. =.DELTA..sub.a .DELTA..sub.e) beam output signals along the azimuth axis. Monopulse ratios along azimuth or elevation direction can then be formed giving the azimuth and elevation DOA estimates by using the following equations: ##EQU3## These derivations make use of the separable property of the planar array patterns.
The monopulse technique for DOA estimation fails when there is sidelobe jamming (SLJ) and/or main lobe jamming (MLJ). If not effectively countered, electronic jamming prevents successful radar target detection and tracking. The situation is exacerbated by introduction of stealth technology to reduce the radar cross section (RCS) of unfriendly aircraft targets. The frequency dependence of the RCS encourages use of lower microwave frequency bands for detection. This leads to large apertures to achieve angular resolution. Large apertures to achieve small beamwidth results in interception of more jamming. On the other hand, constrained apertures lead to wider beamwidth, which implies interception of more mainlobe jamming.
Heretofore, no viable or practical technique for cancelling simultaneous mainlobe and sidelobe jammers has been developed or fielded in a radar. This makes the conception and development of such technique one of the more pressing and critical issues facing radar today. The challenge is to develop adaptive beamforming architectures and signal processing algorithms to cancel mainlobe and sidelobe jammers while maintaining target detection and angle estimation accuracy on mainlobe targets.
Clark (see C. R. Clark, "Main Beam Jammer Cancellation and Target Angle Estimation with a Polarization-Agile Monopulse Antenna," 1989 IEEE Radar Conference, Mar. 29-30, 1989, Dallas, Tex., pp. 95-100) addresses the problem of simultaneous mainlobe and sidelobe jamming cancellation but his work is distinguished from the present invention in three respects. First, Clark does not include the requirement of maintaining the monopulse ratio. Second, his approach uses the main array and sidelobe auxiliary array simultaneously. Third, as a consequence of using the arrays simultaneously, Clark's approach does not include mainlobe maintenance, thereby introducing distortion into the main beam.
It is therefore an object of the invention to adaptively and optimally suppress the jamming of monopulse radar before the sum and difference beam output signals are formed for monopulse processing.
Another object of the invention is to cancel a single mainlobe jammer and multiple sidelobe jammers of monopulse radar while maintaining target detection and angle estimation accuracy on mainlobe targets.
Another object of the invention is to incorporate a sidelobe jamming canceller and a mainlobe jamming canceller in a monopulse radar digital beamforming (DBF) architecture so as to maintain the monopulse accuracy for DOA estimation for mainlobe targets.
According to the basic principles of the invention, jammers of monopulse radar are nulled before forming the final .SIGMA. and .DELTA. beam output signals for monopulse processing. This is accomplished by a filtering approach together with a mainlobe maintenance technique. Identical processing is also required for both the .SIGMA. and .DELTA. beams in order to form an identical set of nulls responsive to the sidelobe jammers.
In a specific implementation of the invention, the sidelobe jammers (SLJs) are first suppressed but not the mainlobe jammer (MLJ). It is essential to include an appropriate mainlobe maintenance (MLM) technique at a prefiltering stage to prevent adverse interaction between the two techniques. The MLM technique prevents the sidelobe cancelling adaptive array technique from interfering with the mainlobe canceller (MLC). The resulting beams are adapted using Applebaum's orthogonal nulling technique to cancel the mainlobe jammer along each axis while forming the monopulse ratio in the other axis. (See S. P. Applebaum and R. Wasiewicz, Main Beam Jammer Cancellation for Monopulse Sensors, Final Tech. Report DTIC RADC-TR-86-267, December, 1984.)
In accordance with a preferred embodiment of the invention, a monopulse radar system is provided having an adaptive array antenna, a mainlobe canceller, and a monopulse processor for determining angle of arrival, the adaptive array antenna comprising multiple elemental sensors, the monopulse processor estimating angle of arrival using sum and difference beam output signals, and the mainlobe canceller generating the sum and difference beam output signals which, for one class of rectangular array with independent horizontal and vertical beamforming, can be expressed as a product of elevation and azimuth factors for use by the monopulse processor, simultaneously yielding an undistorted elevation angular measurement by cancelling a mainlobe jammer with nulls in azimuth and an undistorted azimuth angular measurement by cancelling the mainlobe jammer with nulls in elevation. Preprocessing means are coupled to the adaptive array antenna for forming an identical set of nulls responsive to jammers before the sum and difference beam output signals are formed for monopulse processing. The preprocessing comprises means for applying adaptive weights to the measured signal for suppression of sidelobe jamming. Means are provided for generating adaptive weight using a sample matrix inverse estimate with appropriate mainlobe maintenance. Additional means are provided for maintaining the mainlobe during preprocessing and still further means are provided for coupling the adaptive array in cascade with the mainlobe canceller.